Recombinant DNA techniques have permitted the commercial production of medically useful quantities of therapeutic polypeptides. The short half-life of many therapeutic polypeptides, however, has historically posed a challenge to the effective administration of these compounds. There are several commercially important polypeptide-based drugs currently in use which would benefit from increased half-life: Erythropoietin (t1/2=180, min), insulin (t1/2=5 min), interferon α-2b (t1/2=120 min), interferon β (t1/2=60 min), interferon γ (t1/2=30 min), granulocyte colony stimulating factor (t1/2=120 min), human growth hormone (t1/2=30 min), granulocyte macrophage colony stimulating factor (t1/2=120 min), relaxin (t1/2=30 min), urokinase (t1/2=50 min), streptokinase (t1/2=80 min), tissue plasminogen activator (t1/2=55 min), and tumor necrosis factor (t1/2=18 min). Extending the half-life of therapeutic polypeptides can improve current therapies by allowing dosing amounts and frequency of dosing to be reduced.